Overview on the spatial–temporal characteristics of the ozone formation regime in China

Ozone (O₃), a main component in photochemical smog, is a secondary pollutant formed through complex photochemical reactions involving nitrogen oxides (NOₓ) and volatile organic compounds (VOCs). In the past few decades, with the rapid economic development, industrialization and urbanization, the mixing ratio of O₃ has increased substantially in China. O₃ non-attainment days have been frequently observed. Despite great efforts made in the past few years, it is still difficult to alleviate O₃ pollution in China, due to its non-linear relationship with the precursors. In view of the severe situation in China, this study presents a comprehensive review on the spatial–temporal variations of the relationship between O₃ and its precursors (i.e. O₃ formation regime), built upon the previous reviews of the spatial–temporal variations of O₃ and its precursor levels. Valuable findings from previous studies are laid out for a better understanding of O₃ pollution, followed by implications for the control of O₃ pollution. This literature review indicates that O₃ formation in most areas of the North China Plain (NCP), Yangtze River Delta (YRD) and Pearl River Delta (PRD) regions is in a VOC-limited regime during the high-O₃ seasons due to dramatic emissions from human activities in cities. Outside these metropolitan areas, a NOₓ-limited regime dominates rural/remote areas. From summer to winter, the O₃ formation regime over China shows a tendency to shift to a VOC-limited regime. Furthermore, O₃ formation in China shifted toward increasing sensitivity to VOC emissions before the 12ᵗʰ Five-Year-Plan. However, after the 12ᵗʰ Five-Year-Plan, successful reduction of NOₓ slowed down this trend. Further effective control of VOCs is expected to achieve sustained O₃ attainment in the future. To timely solve the current O₃ pollution problem, precise control of O₃ precursors is proposed, together with the joint prevention and control of regional air pollution.